Band reject filters may be used in wireless network equipment, such as in base stations. Desirably, these band reject filters should be miniature in size to reduce the overall size of equipment in which they are installed. Surface acoustic wave (SAW), bulk acoustic wave (BAW) and film bulk acoustic resonators (FBAR) have been used to implement miniature band pass filters, but have not been widely used in band reject filter implementations, primarily due to a lack of demand for such implementations in wireless network equipment that are based on first generation (1G) and second generation (2G) wireless communication standards.
However, with the emergence of implementations of third generation (3G) and fourth generation (4G) wireless communication standards, frequency spectrum allocation is constrained, requiring very closely spaced frequency channels. This means that filters in a radio frequency (RF) front end must have steep transition bands to avoid interference. Steep transitions can be achieved with band reject filters having high Q. However, when high Q band reject filters are implemented using conventional components, such as air cavity filters, such filters are undesirably large in size and are relatively expensive.
Acoustic wave filters such as the SAW, BAW and FBAR filters, have been used to implement band pass filters, but not miniature band reject filters having steep transitions bands and deep reject bands.